1. Field of the Invention
The present invention relates to an apparatus and method for the analysis of photopolymerization, a photopolymerization process and a photoinitiating composition. More particularly, it relates to an apparatus and method for photopolymerization analysis and a process for the photopolymerization of ethylenically-unsaturated organic compounds containing a photoinitiating system comprising a sensitizer of naphthalene or substituted naphthalene and an organic peroxygen-containing compound. It also relates to a photoinitiating composition comprising a peroxygen-containing naphthalenic compound.
2. Description of the Prior Art
The use of a variety of sensitizers in photochemical polymerization of monomers or mixtures of unsaturated polymers and monomers copolymerized with the latter is well known to those skilled in the art. Benzoin-derived sensitizers are among the most efficient for UV sensitive systems. For example, U.S. Pat. No. 2,448,828 describes the photopolymerization of a number of ethylenically-unsaturated compounds, such as acrylic and substituted acrylic acids, esters, amides, and nitriles, as well as vinyl and vinylidene compounds using benzoin C.sub.1 to C.sub.3 alkyl ethers as photoinitiators. The systems are noted to be activated at 180-700 millimicrons. Molding and coating compositions obtained from mixtures of unsaturated polyester resins and copolymerizable monomers have also been photopolymerized, as described in U.S. Pat. No. 3,502,487. Successful polymerizations occur at 250-500 millimicrons in the presence of certain benzoin aryl ethers. Other benzoin-derived photosensitizers are disclosed in U.S. Pat. Nos. 3,764,560; 3,814,702; and British Specification No. 1,156,460.
Historically, the use of photoinitiators such as the before-mentioned benzoin and its derivatives in photopolymerizable systems has been to facilitate the initiation of the polymerization reaction. Photoinitiators offer the advantage of enhanced curing speed at ambient conditions as opposed to conventionally-used thermal initiators, which require elevated temperatures for use. These thermal initiators are typically free-radical-generating sources whose function is to contribute catalytically to the polymerization reaction by enhancing the rate at which free-radical chains are initiated or by shortening the induction period for the reaction. The most acceptable of these free-radical-generating sources are those classified generally as peroxide initiators, including various organic peracids, peresters, percarbonates, peroxides and hydroperoxides, and azo initiators such as azoisobutyronitrile and the like. While desirable as initiators, these materials require that the polymerizable material in which they are incorporated be heated to an elevated temperature causing decomposition of the initiator, generating free radicals at a rate that will cause polymerization in a satisfactory period of time. For application on thermally-sensitive substrates, the use of thermal initiators may be undesirable. The use of thermal initiators that decompose to free-radical moieties at temperatures near ambient are also useful in polymerization reactions, but present a serious disadvantage in that the storage and shelf life of such composition is minimal.
In addition to the well-known benzoin initiators, haloalkyl-substituted naphthalenic derivatives have been disclosed as photochemical polymerization catalysts in U.S. Pat. No. 2,505,067.
While the above compounds are useful for the purpose for which they are intended, they have the above-mentioned limitations. In addition, many of the conventional photoinitiators bear chromophoric groups or form chromophoric groups during the course of polymerization that makes their use in uncolored, translucent or transparent resins disadvantageous. Further, many of the conventional photoinitiators are not effective as thermal polymerization initiators.
The analysis of thermally-polymerizing systems is a common practice among those skilled in the art of studying such polymerization processes, and methods and apparatus are available. In particular, thermal analyzers which measure exotherms of polymerizing systems are commonly used. The use of such methodology with photopolymerizing systems is severely restricted because of the requirements for supplying ultraviolet radiation to the sample in a manner that will both permit full light absorption and proper form, e.g. that the sample will duplicate conditions of actual use, specifically as a thin film in the case of coatings. In addition to the above problems, conventional thermal analyzers also require tedious manipulations to obtain a reproducible sample and do not respond satisfactorily to the very rapid polymerizations that are commonly encountered in the photopolymerization of thin-film coatings. These limitations prevent a completely reliable, accurate analysis of photopolymerizing systems.